<!doctype html>
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<head>
    <title>WinDivert 1.1 Documentation</title>
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<h1>WinDivert 1.1: Windows Packet Divert</h1>
<h2>Table of Contents</h2>
<ul>
<li><a href="#introduction">1. Introduction</a></li>
<li><a href="#building">2. Building</a></li>
<ul>
<li><a href="#driver_signing">2.1 Driver Signing</a></li>
<li><a href="#visual_studio_2012_support">2.2 Visual Studio 2012 Support</a></li>
<li><a href="#mingw_support">2.3 MinGW Support</a></li>
</ul>
<li><a href="#installing">3. Installing</a></li>
<li><a href="#uninstalling">4. Uninstalling</a></li>
<li><a href="#programming_api">5. Programming API</a></li>
<ul>
<li><a href="#divert_address">5.1 WINDIVERT_ADDRESS</a></li>
<li><a href="#divert_open">5.2 WinDivertOpen</a></li>
<li><a href="#divert_recv">5.3 WinDivertRecv</a></li>
<li><a href="#divert_recv_ex">5.4 WinDivertRecvEx</a></li>
<li><a href="#divert_send">5.5 WinDivertSend</a></li>
<li><a href="#divert_send_ex">5.6 WinDivertSendEx</a></li>
<li><a href="#divert_close">5.7 WinDivertClose</a></li>
<li><a href="#divert_set_param">5.8 WinDivertSetParam</a></li>
<li><a href="#divert_get_param">5.9 WinDivertGetParam</a></li>
</ul>
<li><a href="#helper_programming_api">6. Helper Programming API</a></li>
<ul>
<li><a href="#divert_iphdr">6.1 WINDIVERT_IPHDR</a></li>
<li><a href="#divert_ipv6hdr">6.2 WINDIVERT_IPV6HDR</a></li>
<li><a href="#divert_icmphdr">6.3 WINDIVERT_ICMPHDR</a></li>
<li><a href="#divert_icmpv6hdr">6.4 WINDIVERT_ICMPV6HDR</a></li>
<li><a href="#divert_tcphdr">6.5 WINDIVERT_TCPHDR</a></li>
<li><a href="#divert_udphdr">6.6 WINDIVERT_UDPHDR</a></li>
<li><a href="#divert_helper_parse_packet">6.7 WinDivertHelperParsePacket</a></li>
<li><a href="#divert_help_parse_ipv4_address">6.8 WinDivertHelperParseIPv4Address</li>
<li><a href="#divert_help_parse_ipv6_address">6.9 WinDivertHelperParseIPv6Address</li>
<li><a href="#divert_helper_calc_checksums">6.10 WinDivertHelperCalcChecksums</a></li>
</ul>
<li><a href="#filter_language">7. Filter Language</a></li>
<ul>
<li><a href="#filter_examples">7.1 Filter Examples</a></li>
<li><a href="#filter_usage">7.2 Filter Usage</a></li>
</ul>
<li><a href="#samples">8. Samples</a></li>
<li><a href="#known_issues">9. Known Issues</a></li>
<li><a href="#license">10. License</a></li>
</ul>

<hr>
<a name="introduction"><h2>1. Introduction</h2></a>
<p>
WinDivert is a user-mode 
capture/sniffing/modification/blocking/re-injection package for
Windows Vista, Windows Server 2008, Windows 7, and Windows 8.
WinDivert can be used to implement user-mode packet filters, packet sniffers,
firewalls, NAT, VPNs, tunneling applications, etc., without the need to
write kernel-mode code.
</p>
<p>
The main features of the WinDivert are:
<ul>
<li> User-mode packet capture, sniffing, dropping, filtering, modification,
     re-injection, etc.</li>
<li> Simple, high-level, programming API.</li>
<li> Fully documented with sample programs.</li>
<li> Full IPv6 support.</li>
<li> Full loopback (localhost) support.</li>
<li> A modern WDF/WFP driver implementation.</li>
<li> Open source; Licensed under GNU Lesser General Public License (LGPL)
     version 3.
     See the <a href="#license">License</a> for more information.</li>
</ul>
</p><p>
WinDivert provides similar functionality to
<tt>divert</tt> sockets from FreeBSD/MacOS, <tt>NETLINK</tt> sockets from
Linux, and some commercial packages such as <tt>WinPkFilter</tt> for Windows.
WinDivert also supports passive packet sniffing similar to <tt>Winpcap</tt>.
</p>

<hr>
<a name="building"><h2>2. Building</h2></a>
<p>
The source code for WinDivert is available for download at
<blockquote>
<a href="https://github.com/basil00/Divert">
    https://github.com/basil00/Divert</a>
</blockquote>
To build the WinDivert package from source:
<ol>
<li> Download and install <a href="http://www.microsoft.com/whdc/devtools/wdk/default.mspx">
Windows Driver Kit 7.1.0</a>.</li>
<li> Open a <i>Free Build Environment</i> console (or
    <i>Checked Build Environment</i> for debugging).</li>
<li> In the WinDivert package root directory, run the command:
<pre>
wddk-build.bat
</pre>
This will build the following files and place them in the
<tt>install\WDDK</tt> subdirectory:
<ul>
<li> <tt>WinDivert.dll</tt>: User-mode library.</li>
<li> <tt>WinDivert32.sys</tt> or <tt>WinDivert64.sys</tt>:
    Kernel-mode WDF/WFP call-out driver.</li>
<li> <tt>*.exe</tt>: Sample applications from the <tt>divert\examples</tt>
    directory.</li>
</ul></li>
</ol>
<i>NOTE:</i> The <tt>WinDivert.dll</tt> and <tt>WinDivert.lib</tt> files
are only compatible with programs compiled with the WDDK compiler.
See below for Visual Studio 2012 and MinGW support.
</p>

<a name="driver_signing"><h3>2.1 Driver Signing</h3></a>
<p>
Before the WinDivert package can be used, the
<tt>WinDivert32.sys</tt>/<tt>WinDivert64.sys</tt> driver
must be digitally signed.
See <a href="http://msdn.microsoft.com/en-us/windows/hardware/gg487317.aspx">Driver Signing Requirements for Windows</a>
for more information.
</p>

<a name="visual_studio_2012_support"><h3>2.2 Visual Studio 2012 Support</h3></a>
<p>
To build the WinDivert package for Visual Studio 2012:
<ol>
<li> First build the driver by running <tt>wddk-build.bat</tt> as per the
instructions above.</li>
<li> Open a <i>Visual Studio Command Prompt</i> environment.</li>
<li> In the WinDivert package root directory, run the command:
<pre>
msvc-build.bat
</pre>
This will build Visual Studio 2012 compatible files and place them in the
<tt>install\MSVC</tt> subdirectory.</li>
</ol>
</p>

<a name="mingw_support"><h3>2.3 MinGW Support</h3></a>
<p>
To build the WinDivert package for MinGW:
<ol>
<li> First build the driver by running <tt>wddk-build.bat</tt> as per
the instructions above.</li>
<li> In Linux (with the MinGW cross-compilers installed) and in the
WinDivert package root directory, run the command:
<pre>
sh mingw-build.sh
</pre>
This will build MinGW compatible files and place them in the
<tt>install\MINGW</tt> subdirectory.</li>
</ol>

</p>

<hr>
<a name="installing"><h2>3. Installing</h2></a>
<p>
WinDivert does not require any special installation.
Depending on your target configuration, simply place the following files in
your application's home directory:
</p>
<center>
<table border="1" cellpadding="5" width="75%">
<tr>
<th>
Application Type
</th>
<th>
Target Windows Type
</th>
<th>
Files Required
</th>
</tr>
<tr>
<td>
32-bit
</td>
<td>
32-bit Windows only
</td>
<td>
<tt>WinDivert.dll</tt> (32-bit version) and <tt>WinDivert32.sys</tt>
</td>
</tr>
<tr>
<td>
64-bit
</td>
<td>
64-bit Windows only
</td>
<td>
<tt>WinDivert.dll</tt> (64-bit version) and <tt>WinDivert64.sys</tt>
</td>
</tr>
<tr>
<td>
32-bit
</td>
<td>
Both 32-bit and 64-bit Windows
</td>
<td>
<tt>WinDivert.dll</tt> (32-bit version), <tt>WinDivert32.sys</tt>,
    and <tt>WinDivert64.sys</tt>
</td>
</tr>
</table>
</center>
<p>
The WinDivert driver is automatically (and silently) installed on demand
whenever your application calls
<a href="#divert_open"><tt>WinDivertOpen()</tt></a>.
The calling application must have Administrator privileges.
</p>
<p>
The <tt>WinDivert.dll</tt> also depends on a C run-time library.
This is not distributed with the WinDivert binaries, and must be 
installed/distributed separately if required.
</p>
<center>
<table border="1" cellpadding="5" width="75%">
<tr>
<th>
Build/Compiler
</th>
<th>
C-Runtime Dependency
</th>
<th>
Installed on Windows by Default?
</th>
</tr>
<tr>
<td>
WDDK (Windows Driver Kit 7.1)
</td>
<td>
<tt>MSVCRT.dll</tt>
</td>
<td>
Yes
</td>
</tr>
<tr>
<td>
MSVC (Visual Studio 2012)
</td>
<td>
<tt>MSVCRT110.dll</tt>
</td>
<td>
No.
Must be installed separately or included with your application.
</td>
</tr>
<tr>
<td>
MINGW
</td>
<td>
<tt>MSVCRT.dll</tt>
</td>
<td>
Yes
</td>
</tr>
</table>
</center>

<hr>
<a name="uninstalling"><h2>4. Uninstalling</h2></a>
<p>
To uninstall, simply delete the <tt>WinDivert.dll</tt>,
<tt>WinDivert32.sys</tt>, and <tt>WinDivert64.sys</tt> files.
If already running, the WinDivert driver will be automatically
uninstalled during the next machine reboot.
The WinDivert driver can also be manually removed by issuing the following
commands at the command prompt
<pre>
sc stop WinDivert1.1
sc delete WinDivert1.1
</pre>
Note that this is not recommended as it will interfere with other
applications that depend on WinDivert.
</p>

<hr>
<a name="programming_api"><h2>5. Programming API</h2></a>
<p>
To use the WinDivert package, a program/application must:
<ol>
<li> Include the <tt>windivert.h</tt> header file
<pre>
#include "windivert.h"
</pre></li>
<li> Link against or dynamically load the <tt>WinDivert.dll</tt> dynamic link
library.</li>
</ol>

<a name="divert_address"><h3>5.1 WINDIVERT_ADDRESS</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
typedef struct
{
    UINT32 IfIdx;
    UINT32 SubIfIdx;
    UINT8  Direction;
} <b>WINDIVERT_ADDRESS</b>, *<b>PWINDIVERT_ADDRESS</b>;
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Fields</b>
<ul>
<li> <tt>IfIdx</tt>: The interface index on which the packet arrived
    (for inbound packets), or is to be sent (for outbound packets).</li>
<li> <tt>SubIfIdx</tt>: The sub-interface index for <tt>IfIdx</tt>.</li>
<li> <tt>Direction</tt>: The packet's direction.
The possible values are
<ul>
<li> <tt>WINDIVERT_DIRECTION_OUTBOUND</tt> with value 0 for <i>outbound</i>
packets.</li>
<li> <tt>WINDIVERT_DIRECTION_INBOUND</tt> with value 1 for <i>inbound</i>
packets.</li>
</ul></li>
</ul>
</p><p>
<b>Remarks</b><br>
The <tt>WINDIVERT_ADDRESS</tt> structure represents the "address" of a captured
or injected packet.
The address includes the packet's network interfaces and the packet direction.
</p>
</dd></dl>

<a name="divert_open"><h3>5.2 WinDivertOpen</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
HANDLE <b>WinDivertOpen</b>(
    __in const char *filter,
    __in WINDIVERT_LAYER layer,
    __in INT16 priority,
    __in UINT64 flags
);
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Parameters</b><br>
<ul>
<li> <tt>filter</tt>: A packet filter string specified in the WinDivert
    <a href="#filter_language">filter language</a>.</li>
<li> <tt>layer</tt>: The layer.</li>
<li> <tt>priority</tt>: The priority of the handle.</li>
<li> <tt>flags</tt>: Additional flags.</li>
</ul>
</p><p>
<b>Return Value</b><br>
A valid WinDivert handle on success, or
<tt>INVALID_HANDLE_VALUE</tt> if an error occurred.
Use <tt>GetLastError()</tt> to get the reason for the error.
Common errors include:
<center>
<table border="1" cellpadding="5" width="75%">
<tr>
<th>
Name
</th>
<th>
Code
</th>
<th>
Description
</th>
</tr>
<tr>
<td>
<tt>ERROR_FILE_NOT_FOUND</tt>
</td>
<td>
2
</td>
<td>
The driver files
<tt>WinDivert32.sys</tt> or <tt>WinDivert64.sys</tt>
were not found.
</td>
</tr>
<tr>
<td>
<tt>ERROR_ACCESS_DENIED</tt>
</td>
<td>
5
</td>
<td>
The calling application does not have Administrator privileges.
</td>
</tr>
<tr>
<td>
<tt>ERROR_INVALID_PARAMETER</tt>
</td>
<td>
87
</td>
<td>
This indicates an invalid packet filter string, layer, priority, or flags.
</td>
</tr>
<tr>
<td>
<tt>ERROR_INVALID_IMAGE_HASH</tt>
</td>
<td>
577
</td>
<td>
The <tt>WinDivert32.sys</tt> or <tt>WinDivert64.sys</tt> driver does not
have a valid digital signature
(see the <a href="#driver_signing">driver signing requirements</a> above).
</td>
</tr>
<tr>
<td>
<tt>ERROR_DRIVER_BLOCKED</tt>
</td>
<td>
1275
</td>
<td>
This error occurs for various reasons, including:
<ol>
<li> the WinDivert driver is blocked by security software; or</li>
<li> you are using a virtualization environment that does not support
drivers.</li>
</ol>
</td>
</tr>
</table>
</center>
</p><p>
<b>Remarks</b><br>
Opens a WinDivert handle for the given filter.
Unless otherwise specified by <tt>flags</tt>, any packet that matches the
filter will be diverted to the handle.
Diverted packets can be read by the application with
<a href="#divert_recv"><tt>WinDivertRecv()</tt></a>.
</p>
<p>
A typical application is only interested in a subset of all network traffic.
In this case the filter should <i>match as closely as possible</i> to
the subset of interest.
This avoids unnecessary overheads introduced by diverting packets to the
user-mode application.
See the <a href="#filter_language">filter language</a> section for more
information.
</p>
<p>
The <i>layer</i> of the WinDivert handle is determined by the <tt>layer</tt>
parameter.
Currently the following layers are supported.
<center>
<table border="1" cellpadding="5" width="75%">
<tr>
<th>
Layer
</th>
<th>
Description
</th>
</tr>
<tr>
<td>
<tt>WINDIVERT_LAYER_NETWORK = 0</tt>
</td>
<td>
The network layer.
This is the default.
</td>
</tr>
<tr>
<td>
<tt>WINDIVERT_LAYER_NETWORK_FORWARD</tt>
</td>
<td>
The network layer (forwarded packets).
</td>
</tr>
</table>
</center>
</p>
<p>
Different WinDivert handles can be assigned different priorities by the
<tt>priority</tt> parameter.
Packets are diverted to higher priority handles before lower priority
handles.
Packets injected by a handle are then diverted to the next priority handle,
and so on, provided the packet matches the handle's filter.
A packet is only diverted once per priority level, so handles should not
share priority levels unless they use mutually exclusive filters.
Otherwise it is not defined which handle will receive the packet first.
Lower <tt>priority</tt> values represent higher priorities, with <tt>-1000</tt>
being the highest priority, <tt>0</tt> the middle (and a good default)
priority, and <tt>1000</tt> the lowest priority.
</p>
<p>
The following flags are supported.
<center>
<table border="1" cellpadding="5" width="75%">
<tr>
<th>
Flag
</th>
<th>
Description
</th>
</tr>
<tr>
<td>
<tt>WINDIVERT_FLAG_SNIFF</tt>
</td>
<td>
This flag opens the WinDivert handle in <i>packet sniffing</i> mode.
In packet sniffing mode the original packet is not dropped-and-diverted
(the default) but copied-and-diverted.
This mode is useful for implementing packet sniffing tools similar to those
applications that currently use <tt>Winpcap</tt>.
</td>
</tr>
<tr>
<td>
<tt>WINDIVERT_FLAG_DROP</tt>
</td>
<td>
This flag indicates that the user application does not intend to read matching
packets with <a href="#divert_recv"><tt>WinDivertRecv()</tt></a>, instead the
packets should be silently dropped.
This is useful for implementing simple packet filters using the
WinDivert <a href="#filter_language">filter language</a>.
</td>
</tr>
<tr>
<td>
<tt>WINDIVERT_FLAG_NO_CHECKSUM</tt>
</td>
<td>
By default WinDivert ensures that each diverted packet has a valid checksum.
If the checksum is missing (e.g. with TCP checksum offloading), WinDivert
will calculate it before passing the packet to the user application.
This flag disables this behavior.
</table>
</center>
Note that only one of <tt>WINDIVERT_FLAG_SNIFF</tt> or
<tt>WINDIVERT_FLAG_DROP</tt> may be set at the same time.
</p>
</dd></dl>

<a name="divert_recv"><h3>5.3 WinDivertRecv</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
BOOL <b>WinDivertRecv</b>(
    __in HANDLE handle,
    __out PVOID pPacket,
    __in UINT packetLen,
    __out_opt PWINDIVERT_ADDRESS pAddr,
    __out_opt UINT *recvLen
);
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Parameters</b><br>
<ul>
<li> <tt>handle</tt>: A valid WinDivert handle created by
     <a href="#divert_open"><tt>WinDivertOpen()</tt></a>.</li>
<li> <tt>pPacket</tt>: A buffer for the captured packet.</li>
<li> <tt>packetLen</tt>: The length of the buffer <tt>pPacket</tt>.</li>
<li> <tt>pAddr</tt>: The <tt>WINDIVERT_ADDRESS</tt> of the captured packet.</li>
<li> <tt>recvLen</tt>: The total number of bytes written to <tt>pPacket</tt>.
     Can be <tt>NULL</tt> if this information is not required.</li>
</ul>
</p><p>
<b>Return Value</b><br>
<tt>TRUE</tt> if a packet was successfully received, or <tt>FALSE</tt> if
an error occurred.
Use <tt>GetLastError()</tt> to get the reason for the error.
</p><p>
<b>Remarks</b><br>
Receives a diverted packet that matched the filter passed to
<a href="#divert_open"><tt>WinDivertOpen()</tt></a>.
The received packet is guaranteed to match the filter.
</p><p>
The contents of the captured packet are written to <tt>pPacket</tt>.
If the captured packet is larger than the <tt>pPacket</tt> buffer length,
then the packet will be truncated.
If <tt>recvLen</tt> is non-<tt>NULL</tt>, then the total number of bytes
written to <tt>pPacket</tt> is placed there.
If non-<tt>NULL</tt>, the address of the captured packet is written to
<tt>pAddr</tt>.
</p><p>
An application should call <a href="#divert_recv"><tt>WinDivertRecv()</tt></a>
<i>as soon as possible</i>
after a successful call to <a href="#divert_open"><tt>WinDivertOpen()</tt></a>.
When a WinDivert handle is open, any packet that matches the filter will
be captured and queued until handled by
<a href="#divert_recv"><tt>WinDivertRecv()</tt></a>.
Packets are not queued indefinitely, and if not handled in a timely manner,
any captured packet may be dropped.
The amount of time a packet is queued can be controlled with the
<a href="#divert_set_param"><tt>WinDivertSetParam()</tt></a> function.
</p>
<p>
<a href="#divert_recv"><tt>WinDivertRecv()</tt></a> should not be used on any
WinDivert handle created with the <tt>WINDIVERT_FLAG_DROP</tt> set.
</p>
</dd></dl>

<a name="divert_recv_ex"><h3>5.4 WinDivertRecvEx</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
BOOL <b>WinDivertRecvEx</b>(
    __in HANDLE handle,
    __out PVOID pPacket,
    __in UINT packetLen,
    __in UINT64 flags,
    __out_opt PWINDIVERT_ADDRESS pAddr,
    __out_opt UINT *recvLen,
    __inout_opt LPOVERLAPPED lpOverlapped
);
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Parameters</b><br>
<ul>
<li> <tt>handle</tt>: A valid WinDivert handle created by
     <a href="#divert_open"><tt>WinDivertOpen()</tt></a>.</li>
<li> <tt>pPacket</tt>: A buffer for the captured packet.</li>
<li> <tt>packetLen</tt>: The length of the buffer <tt>pPacket</tt>.</li>
<li> <tt>flags</tt>: Reserved, set to zero.</li>
<li> <tt>pAddr</tt>: The <tt>WINDIVERT_ADDRESS</tt> of the captured packet.</li>
<li> <tt>recvLen</tt>: The total number of bytes written to <tt>pPacket</tt>.
     Can be <tt>NULL</tt> if this information is not required.</li>
<li> <tt>lpOverlapped</tt>: An optional pointer to a <tt>OVERLAPPED</tt>
     structure.</li>
</ul>
</p><p>
<b>Return Value</b><br>
<tt>TRUE</tt> if a packet was successfully received, or <tt>FALSE</tt> 
otherwise.
Use <tt>GetLastError()</tt> to get the reason.
The error code <tt>ERROR_IO_PENDING</tt> indicates that the overlapped
operation has been successfully initiated and that completion will be
indicated at a later time.
All other codes indicate an error.
</p><p>
<b>Remarks</b><br>
This function is equivalent to
<a href="#divert_recv"><tt>WinDivertRecv()</tt></a> except that it
supports overlapped I/O via the <tt>lpOverlapped</tt> parameter.
</p>
</dd></dl>

<a name="divert_send"><h3>5.5 WinDivertSend</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
BOOL <b>WinDivertSend</b>(
    __in HANDLE handle,
    __in PVOID pPacket,
    __in UINT packetLen,
    __in PWINDIVERT_ADDRESS pAddr,
    __out_opt UINT *sendLen
);
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Parameters</b><br>
<ul>
<li> <tt>handle</tt>: A valid WinDivert handle created by
     <a href="#divert_open"><tt>WinDivertOpen()</tt></a>.</li>
<li> <tt>pPacket</tt>: A buffer containing the packet to be injected.</li>
<li> <tt>packetLen</tt>: The total length of the buffer <tt>pPacket</tt>.</li> 
<li> <tt>pAddr</tt>: The <tt>WINDIVERT_ADDRESS</tt> for the injected packet.</li>
<li> <tt>sendLen</tt>: The total number of bytes injected.
     Can be <tt>NULL</tt> if this information is not required.</li>
</ul>
</p><p>
<b>Return Value</b><br>
<tt>TRUE</tt> if a packet was successfully injected, or <tt>FALSE</tt> if
an error occurred.
Use <tt>GetLastError()</tt> to get the reason for the error.
</p><p>
<b>Remarks</b><br>
Injects a packet into the network stack.
The injected packet may be one received from
<a href="#divert_recv"><tt>WinDivertRecv()</tt></a>, or a
modified version, or a completely new packet.
Injected packets can be captured and diverted again by other WinDivert
handles with lower priorities.
</p><p>
The <tt>pAddr</tt> parameter determines how the packet is injected.
If the <tt>Direction</tt> field is <tt>WINDIVERT_DIRECTION_OUTBOUND</tt>,
the packet is injected into the <i>outbound</i> path (i.e. a packet leaving
this computer).
Else, if <tt>Direction</tt> is <tt>WINDIVERT_DIRECTION_INBOUND</tt>,
the packet is injected into the <i>inbound</i> path (i.e. a packet arriving at
this computer).
Note that the <tt>Direction</tt> field, and <i>not</i> the IP addresses in 
the injected packet, is used to determine the packet's direction.
</p><p>
For packets injected into the <i>inbound</i> path, the <tt>IfIdx</tt> and
<tt>SubIfIdx</tt> fields are assumed to contain valid interface numbers.
These may be retrieved from <a href="#divert_recv"><tt>WinDivertRecv()</tt></a>
(for packet modification),
or from the <a href="http://msdn.microsoft.com/en-us/library/aa366073%28v=VS.85%29.aspx">IP Helper API</a>.
</p><p>
For <i>outbound</i> injected packets, the <tt>IfIdx</tt> and <tt>SubIfIdx</tt>
fields are currently ignored and may be arbitrary values.
Injecting an inbound packet on the outbound path <i>may</i> work (for some
types of packets), however this should be considered "undocumented" behavior,
and may be changed in the future.
</p>
</dd></dl>

<a name="divert_send_ex"><h3>5.6 WinDivertSendEx</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
BOOL <b>WinDivertSendEx</b>(
    __in HANDLE handle,
    __in PVOID pPacket,
    __in UINT packetLen,
    __in UINT64 flags,
    __in PWINDIVERT_ADDRESS pAddr,
    __out_opt UINT *sendLen,
    __inout_opt LPOVERLAPPED lpOverlapped
);
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Parameters</b><br>
<ul>
<li> <tt>handle</tt>: A valid WinDivert handle created by
     <a href="#divert_open"><tt>WinDivertOpen()</tt></a>.</li>
<li> <tt>pPacket</tt>: A buffer containing the packet to be injected.</li>
<li> <tt>packetLen</tt>: The total length of the buffer <tt>pPacket</tt>.</li>
<li> <tt>flags</tt>: Reserved, set to zero.</li>
<li> <tt>pAddr</tt>: The <tt>WINDIVERT_ADDRESS</tt> for the injected packet.</li>
<li> <tt>sendLen</tt>: The total number of bytes injected.
     Can be <tt>NULL</tt> if this information is not required.</li>
<li> <tt>lpOverlapped</tt>: An optional pointer to a <tt>OVERLAPPED</tt>
     structure.</li>
</ul>
</p><p>
<b>Return Value</b><br>
<tt>TRUE</tt> if a packet was successfully injected, or <tt>FALSE</tt>
otherwise.
Use <tt>GetLastError()</tt> to get the reason.
The error code <tt>ERROR_IO_PENDING</tt> indicates that the overlapped
operation has been successfully initiated and that completion will be
indicated at a later time.
All other codes indicate an error.
</p><p>
<b>Remarks</b><br>
This function is equivalent to
<a href="#divert_send"><tt>WinDivertSend()</tt></a> except that it
supports overlapped I/O via the <tt>lpOverlapped</tt> parameter.
</p>
</dd></dl>

<a name="divert_close"><h3>5.7 WinDivertClose</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
BOOL <b>WinDivertClose</b>(
    __in HANDLE handle
);
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Parameters</b><br>
<ul>
<li> <tt>handle</tt>: A valid WinDivert handle created by
     <a href="#divert_open"><tt>WinDivertOpen()</tt></a>.</li>
</ul>
</p><p>
<b>Return Value</b><br>
<tt>TRUE</tt> if successful, <tt>FALSE</tt> if an error occurred.
Use <tt>GetLastError()</tt> to get the reason for the error.
</p><p>
<b>Remarks</b><br>
Closes a WinDivert handle created by
<a href="#divert_open"><tt>WinDivertOpen()</tt></a>.
</p>
<dd></dl>

<a name="divert_set_param"><h3>5.8 WinDivertSetParam</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
BOOL <b>WinDivertSetParam</b>(
    __in HANDLE handle,
    __in WINDIVERT_PARAM param,
    __in UINT64 value);
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Parameters</b><br>
<ul>
<li> <tt>handle</tt>: A valid WinDivert handle created by
     <a href="#divert_open"><tt>WinDivertOpen()</tt></a>.</li>
<li> <tt>param</tt>: A WinDivert parameter name.</li>
<li> <tt>value</tt>: The parameter's new value.</li>
</ul>
</p><p>
<b>Return Value</b><br>
<tt>TRUE</tt> if successful, <tt>FALSE</tt> if an error occurred.
Use <tt>GetLastError()</tt> to get the reason for the error.
</p><p>
<b>Remarks</b><br>
Sets a WinDivert parameter.
Currently, the following WinDivert parameters are defined.
<center>
<table border="1" cellpadding="5" width="75%">
<tr>
<th>
Parameter
</th>
<th>
Description
</th>
</tr>
<tr>
<td>
<tt>WINDIVERT_PARAM_QUEUE_LEN</tt>
</td>
<td>
Sets the maximum length of the packet queue for
<a href="#divert_recv"><tt>WinDivertRecv()</tt></a>.
Currently the default value is 512, the minimum is 1, and the maximum 
is 8192.
</td>
</tr>
<tr>
<td>
<tt>WINDIVERT_PARAM_QUEUE_TIME</tt>
</td>
<td>
Sets the minimum time, in milliseconds, a packet can be queued before it is
automatically dropped.
Packets cannot be queued indefinitely, and ideally, packets should be
processed by the application as soon as is possible.
Note that this sets the <i>minimum</i> time a packet can be queued before 
it can be dropped.
The actual time may be exceed this value.
Currently the default value is 512, the minimum is 128, and the maximum is
2048.
</td>
</tr>
</table>
</center>
</p>
<dd></dl>

<a name="divert_get_param"><h3>5.9 WinDivertGetParam</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
BOOL <b>WinDivertGetParam</b>(
    __in HANDLE handle,
    __in WINDIVERT_PARAM param,
    __out UINT64 *pValue);
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Parameters</b><br>
<ul>
<li> <tt>handle</tt>: A valid WinDivert handle created by
     <a href="#divert_open"><tt>WinDivertOpen()</tt></a>.</li>
<li> <tt>param</tt>: A WinDivert parameter name.</li>
<li> <tt>value</tt>: The parameter's current value.</li>
</ul>
</p><p>
<b>Return Value</b><br>
<tt>TRUE</tt> if successful, <tt>FALSE</tt> if an error occurred.
Use <tt>GetLastError()</tt> to get the reason for the error.
</p><p>
<b>Remarks</b><br>
Gets a WinDivert parameter.
See <a href="#divert_set_param"><tt>WinDivertSetParam()</tt></a> for the list
of parameters.
</p>
<dd></dl>

<hr>
<a name="helper_programming_api"><h2>6. Helper Programming API</h2></a>

The WinDivert helper programming API is a collection of definitions
and functions designed to make writing WinDivert applications easier.
The use of the helper API is completely optional.

<a name="divert_iphdr"><h3>6.1 WINDIVERT_IPHDR</h3>
<table border="1" cellpadding="5"><tr><td>
<pre>
typedef struct
{
    UINT8  HdrLength:4;
    UINT8  Version:4;
    UINT8  TOS;
    UINT16 Length;
    UINT16 Id;
    UINT16 <i>...</i>;
    UINT8  TTL;
    UINT8  Protocol;
    UINT16 Checksum;
    UINT32 SrcAddr;
    UINT32 DstAddr;
} <b>WINDIVERT_IPHDR</b>, *<b>PWINDIVERT_IPHDR</b>;
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Fields</b><br>
See <a href="http://en.wikipedia.org/wiki/IPv4#Packet_structure">here</a>
for more information.
</p><p>
<b>Remarks</b><br>
IPv4 header definition.
</p><p>
The following fields can only be get/set using the following macro
definitions:
<ul>
<li><i>FragOff</i> with <tt>WINDIVERT_IPHDR_GET_FRAGOFF(<i>hdr</i>)</tt> and
    <tt>WINDIVERT_IPHDR_SET_FRAGOFF(<i>hdr</i>, <i>val</i>)</tt></li>
<li><i>MF</i> with <tt>WINDIVERT_IPHDR_GET_MF(<i>hdr</i>)</tt> and
    <tt>WINDIVERT_IPHDR_SET_MF(<i>hdr</i>, <i>val</i>)</tt></li>
<li><i>DF</i> with <tt>WINDIVERT_IPHDR_GET_DF(<i>hdr</i>)</tt> and
    <tt>WINDIVERT_IPHDR_SET_DF(<i>hdr</i>, <i>val</i>)</tt></li>
<li><i>Reserved</i> with <tt>WINDIVERT_IPHDR_GET_RESERVED(<i>hdr</i>)</tt> and
    <tt>WINDIVERT_IPHDR_SET_RESERVED(<i>hdr</i>, <i>val</i>)</tt></li>
</ul>
</p>
</dl></dd>

<a name="divert_ipv6hdr"><h3>6.2 WINDIVERT_IPV6HDR</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
typedef struct
{
    UINT32 Version:4;
    UINT32 ...:28;
    UINT16 Length;
    UINT8  NextHdr;
    UINT8  HopLimit;
    UINT32 SrcAddr[4];
    UINT32 DstAddr[4];
} <b>WINDIVERT_IPV6HDR</b>, *<b>PWINDIVERT_IPV6HDR</b>;
</pre>
</td></tr></table>
<dl><dd>
<b>Fields</b><br>
See <a href="http://en.wikipedia.org/wiki/IPv6_packet#Fixed_header">here</a>
for more information.
</p><p>
<b>Remarks</b><br>
IPv6 header definition.
</p><p>
The following fields can only be get/set using the following macro
definitions:
<ul>
<li><i>TrafficClass</i> with
    <tt>WINDIVERT_IPV6HDR_GET_TRAFFICCLASS(<i>hdr</i>)</tt> and
    <tt>WINDIVERT_IPV6HDR_SET_TRAFFICCLASS(<i>hdr</i>, <i>val</i>)</tt></li>
<li><i>FlowLabel</i> with <tt>WINDIVERT_IPV6HDR_GET_FLOWLABEL(<i>hdr</i>)</tt> and
    <tt>WINDIVERT_IPV6HDR_SET_FLOWLABEL(<i>hdr</i>, <i>val</i>)</tt></li>
</ul>
</p>
</dl></dd>

<a name="divert_icmphdr"><h3>6.3 WINDIVERT_ICMPHDR</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
typedef struct
{
    UINT8  Type;
    UINT8  Code;
    UINT16 Checksum;
    UINT32 Body;
} <b>WINDIVERT_ICMPHDR</b>, *<b>PWINDIVERT_ICMPHDR</b>;
</pre>
</td></tr></table>
<dl><dd>
<b>Fields</b><br>
See <a href="http://en.wikipedia.org/wiki/Internet_Control_Message_Protocol#ICMP_segment_structure">here</a>
for more information.
</p><p>
<b>Remarks</b><br>
ICMP header definition.
</p>
</dl></dd>

<a name="divert_icmpv6hdr"><h3>6.4 WINDIVERT_ICMPV6HDR</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
typedef struct
{
    UINT8  Type;
    UINT8  Code;
    UINT16 Checksum;
    UINT32 Body;
} <b>WINDIVERT_ICMPV6HDR</b>, *<b>PWINDIVERT_ICMPV6HDR</b>;
</pre>
</td></tr></table>
<dl><dd>
<b>Fields</b><br>
See <a href="http://en.wikipedia.org/wiki/ICMPv6#Packet_format">here</a> for
more information.
</p><p>
<b>Remarks</b><br>
ICMPv6 header definition.
</p>
</dl></dd>

<a name="divert_tcphdr"><h3>6.5 WINDIVERT_TCPHDR</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
typedef struct
{
    UINT16 SrcPort;
    UINT16 DstPort;
    UINT32 SeqNum;
    UINT32 AckNum;
    UINT16 Reserved1:4;
    UINT16 HdrLength:4;
    UINT16 Fin:1;
    UINT16 Syn:1;
    UINT16 Rst:1;
    UINT16 Psh:1;
    UINT16 Ack:1;
    UINT16 Urg:1;
    UINT16 Reserved2:2;
    UINT16 Window;
    UINT16 Checksum;
    UINT16 UrgPtr;
} <b>WINDIVERT_TCPHDR</b>, *<b>PWINDIVERT_TCPHDR</b>;
</pre>
</td></tr></table>
<dl><dd>
<b>Fields</b><br>
See <a href="http://en.wikipedia.org/wiki/Transmission_Control_Protocol#TCP_segment_structure">here</a>
for more information.
</p><p>
<b>Remarks</b><br>
TCP header definition.
</p>
</dl></dd>

<a name="divert_udphdr"><h3>6.6 WINDIVERT_UDPHDR</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
typedef struct
{
    UINT16 SrcPort;
    UINT16 DstPort;
    UINT16 Length;
    UINT16 Checksum;
} <b>WINDIVERT_UDPHDR</b>, *<b>PWINDIVERT_UDPHDR</b>;
</pre>
</td></tr></table>
<dl><dd>
<b>Fields</b><br>
See <a href="http://en.wikipedia.org/wiki/User_Datagram_Protocol#Packet_structure">here</a>
for more information.
</p><p>
<b>Remarks</b><br>
UDP header definition.
</p>
</dl></dd>

<a name="divert_helper_parse_packet"><h3>6.7 WinDivertHelperParsePacket</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
BOOL <b>WinDivertHelperParsePacket</b>(
    __in PVOID pPacket,
    __in UINT packetLen,
    __out_opt PWINDIVERT_IPHDR *ppIpHdr,
    __out_opt PWINDIVERT_IPV6HDR *ppIpv6Hdr,
    __out_opt PWINDIVERT_ICMPHDR *ppIcmpHdr,
    __out_opt PWINDIVERT_ICMPV6HDR *ppIcmpv6Hdr,
    __out_opt PWINDIVERT_TCPHDR *ppTcpHdr,
    __out_opt PWINDIVERT_UDPHDR *ppUdpHdr,
    __out_opt PVOID *ppData,
    __out_opt UINT *pDataLen
);
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Parameters</b><br>
<ul>
<li> <tt>pPacket</tt>: The packet to be parsed.</li>
<li> <tt>packetLen</tt>: The total length of the packet <tt>pPacket</tt>.</li>
<li> <tt>ppIpHdr</tt>: Output pointer to a <tt>WINDIVERT_IPHDR</tt>.</li>
<li> <tt>ppIpv6Hdr</tt>: Output pointer to a <tt>WINDIVERT_IPV6HDR</tt>.</li>
<li> <tt>ppIcmpHdr</tt>: Output pointer to a <tt>WINDIVERT_ICMPHDR</tt>.</li>
<li> <tt>ppIcmpv6Hdr</tt>: Output pointer to a <tt>WINDIVERT_ICMPV6HDR</tt>.</li>
<li> <tt>ppTcpHdr</tt>: Output pointer to a <tt>WINDIVERT_TCPHDR</tt>.</li>
<li> <tt>ppUdpHdr</tt>: Output pointer to a <tt>WINDIVERT_UDPHDR</tt>.</li>
<li> <tt>ppData</tt>: Output pointer to the packet's data/payload.</li>
<li> <tt>pDataLen</tt> Output data/payload length.</li>
</ul>
</p><p>
<b>Return Value</b><br>
<tt>TRUE</tt> if all expected (non-<tt>NULL</tt>) outputs were present,
<tt>FALSE</tt> otherwise.
Note that <tt>FALSE</tt> may sometimes be a legitimate return value, e.g.,
when both <tt>ppIpHdr</tt> and <tt>ppIpv6Hdr</tt> are non-<tt>NULL</tt>.
</p><p>
<b>Remarks</b><br>
Parses a raw packet (e.g. from <a
href="#divert_recv"><tt>WinDivertRecv()</tt></a>) into the
various packet headers and/or payloads that may or may not be present.
</p><p>
Each output parameter may be <tt>NULL</tt> or non-<tt>NULL</tt>.
For non-<tt>NULL</tt> parameters, this function will write the pointer to
the corresponding header/payload if it exists, or will write <tt>NULL</tt>
otherwise.
Any non-<tt>NULL</tt> pointer that is returned
<ol>
<li> Is a pointer into the original <tt>pPacket</tt> packet; and</li>
<li> There is enough space in <tt>pPacket</tt> to fit the header.</li>
</ol>
</p><p>
This function does not do any verification of the header/payload contents
beyond checking the header length and any other minimal information required
for parsing.
<p>
</dd></dl>

<a name="divert_help_parse_ipv4_address"><h3>6.8 WinDivertHelperParseIPv4Address</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
BOOL <b>WinDivertHelperParseIPv4Address</b>(
    __in const char *addrStr,
    __out_opt UINT32 *pAddr
);
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Parameters</b><br>
<ul>
<li> <tt>addrStr</tt>: The address string.</li>
<li> <tt>pAddr</tt>: Output address.</li>
</ul>
</p><p>
<b>Return Value</b><br>
<tt>TRUE</tt> if successful, <tt>FALSE</tt> if an error occurred.
Use <tt>GetLastError()</tt> to get the reason for the error.
</p><p>
<b>Remarks</b><br>
Parses an IPv4 address stored in <tt>addrStr</tt>.
If non-<tt>NULL</tt>, the result is stored in <tt>pAddr</tt>
in host-byte-order.
Use <tt>htonl()</tt> to convert the result into network-byte-order.
</p>
<dd></dl>

<a name="divert_help_parse_ipv6_address"><h3>6.9 WinDivertHelperParseIPv6Address</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
BOOL <b>WinDivertHelperParseIPv6Address</b>(
    __in const char *addrStr,
    __out_opt UINT32 *pAddr
);
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Parameters</b><br>
<ul>
<li> <tt>addrStr</tt>: The address string.</li>
<li> <tt>pAddr</tt>: Output address.</li>
</ul>
</p><p>
<b>Return Value</b><br>
<tt>TRUE</tt> if successful, <tt>FALSE</tt> if an error occurred.
Use <tt>GetLastError()</tt> to get the reason for the error.
</p><p>
<b>Remarks</b><br>
Parses an IPv6 address stored in <tt>addrStr</tt>.
If non-<tt>NULL</tt>, the result is stored in <tt>pAddr</tt>.
The <tt>pAddr</tt> parameter is assumed to point to a buffer large enough
to hold a 16-byte IPv6 address.
Given an IPv6 address of the form
<tt>0011:2233:4455:6677:8899:aabb:ccdd:eeff</tt>, then
the result is ordered as follows:
<pre>
pAddr[0] = 0x00112233
pAddr[1] = 0x44556677
pAddr[2] = 0x8899aabb
pAddr[3] = 0xccddeeff
</pre>
where each <tt>pAddr[i]</tt> is in host-byte-order.
The result can be converted into network-byte-order by setting
<tt>pAddr[i] = htonl(pAddr[i])</tt> for each <tt>i</tt>.
</p>
<dd></dl>

<a name="divert_helper_calc_checksums"><h3>6.10 WinDivertHelperCalcChecksums</h3></a>
<table border="1" cellpadding="5"><tr><td>
<pre>
UINT <b>WinDivertHelperCalcChecksums</b>(
    __inout PVOID pPacket,
    __in UINT packetLen,
    __in UINT64 flags
);
</pre>
</td></tr></table>
<dl><dd>
<p>
<b>Parameters</b><br>
<ul>
<li> <tt>pPacket</tt>: The packet to be modified.</li>
<li> <tt>packetLen</tt>: The total length of the packet <tt>pPacket</tt>.</li>
<li> <tt>flags</tt>: One or more of the following flags:
<ul>
<li> <tt>WINDIVERT_HELPER_NO_IP_CHECKSUM</tt>: Do not calculate the IPv4
    checksum.</li>
<li> <tt>WINDIVERT_HELPER_NO_ICMP_CHECKSUM</tt>: Do not calculate the ICMP
    checksum.</li>
<li> <tt>WINDIVERT_HELPER_NO_ICMPV6_CHECKSUM</tt>: Do not calculate the ICMPv6
    checksum.</li>
<li> <tt>WINDIVERT_HELPER_NO_TCP_CHECKSUM</tt>: Do not calculate the TCP
    checksum.</li>
<li> <tt>WINDIVERT_HELPER_NO_UDP_CHECKSUM</tt>: Do not calculate the UDP
    checksum.</li>
</ul></li>
</ul>
</p><p>
<b>Return Value</b><br>
The number of checksums calculated.
</p><p>
<b>Remarks</b><br>
(Re)calculates the checksum for any IPv4/ICMP/ICMPv6/TCP/UDP checksum present
in the given packet.
Individual checksum calculations may be disabled via the appropriate flag.
Typically this function should be invoked on a modified packet before it is
injected with <a href="#divert_send"><tt>WinDivertSend()</tt></a>.
</p><p>
This function will calculate each checksum from scratch, even if the existing
checksum is correct.
This may be inefficient for some applications.
For better performance, incremental checksum calculations should be used
instead (not provided by this API).
<p>
</dd></dl>

<hr>
<a name="filter_language"><h2>7. Filter Language</h2></a>

<p>
The <a href="#divert_open"><tt>WinDivertOpen()</tt></a> function accepts a
string containing a <i>filter expression</i>.
Only packets that match the filter expression are diverted.
Any other packet is allowed to continue as per normal.
</p><p>
Filter allows an application to select only the subset of traffic that is of
interest.
For example, a URL blacklist filter would only be interested in packets that
contain URLs.
This could be achieved via the following filter.
<pre>
HANDLE handle = WinDivertOpen(
    "outbound and "
    "tcp.PayloadLength &gt; 0 and "
    "tcp.DstPort == 80", 0, 0, 0);
</pre>
This filter specifies that we should only divert traffic that is
<ol>
<li>outbound;</li>
<li>contains a non-empty payload; and</li>
<li>has TCP destination port 80 (i.e. HTTP web traffic).
</ol>
</p><p>
A <i>filter</i> is a Boolean expression of the form:
<pre>
        <i>FILTER</i> := true | false | <i>FILTER</i> and <i>FILTER</i> | <i>FILTER</i> or <i>FILTER</i> | (<i>FILTER</i>) | <i>TEST</i>
</pre>
C-style syntax <tt>&amp;&amp;</tt>, <tt>||</tt>, and <tt>!</tt> may also
be used instead of <tt>and</tt>, <tt>or</tt>, and <tt>not</tt>, respectively.
A <i>test</i> is of the following form:
<pre>
        <i>TEST</i> := <i>TEST0</i> | not <i>TEST0</i>
        <i>TEST0</i> := <i>FIELD</i> | <i>FIELD</i> op <i>VAL</i>
</pre>
where <tt>op</tt> is one of the following:
</p><p>
<center>
<table border="1" cellpadding="5">
<tr><th>Operator</th><th>Description</th></tr>
<tr><td><tt>==</tt> or <tt>=</tt></td><td>Equal</td></tr>
<tr><td><tt>!=</tt></td><td>Not equal</td></tr>
<tr><td><tt>&lt;</tt></td><td>Less-than</td></tr>
<tr><td><tt>&gt;</tt></td><td>Greater-than</td></tr>
<tr><td><tt>&lt;=</tt></td><td>Less-than-or-equal</td></tr>
<tr><td><tt>&gt;=</tt></td><td>Greater-than-or-equal</td></tr>
</table>
</center>
</p><p>
and <tt><i>VAL</i></tt> is a decimal number, hexadecimal number, or IP
address.
If the "<tt>op <i>VAL</i></tt>" is missing, the test is implicitly
"<tt><i>FIELD</i> != 0</tt>".
</p><p>
Finally a <i>field</i> is some property about the packet.
The possible fields are:
</p><p>
<center>
<table border="1" cellpadding="5">
<tr><th>Field</th><th>Description</th></tr>
<tr><td><tt>outbound</tt></td><td>Is outbound?</td></tr>
<tr><td><tt>inbound</tt></td><td>Is inbound?</td></tr>
<tr><td><tt>ifIdx</tt></td><td>Interface index</td></tr>
<tr><td><tt>subIfIdx</tt></td><td>Sub-interface index</td></tr>
<tr><td><tt>ip</tt></td><td>Is IPv4?</td></tr>
<tr><td><tt>ipv6</tt></td><td>Is IPv6?</td></tr>
<tr><td><tt>icmp</tt></td><td>Is ICMP?</td></tr>
<tr><td><tt>icmpv6</tt></td><td>Is ICMPv6?</td></tr>
<tr><td><tt>tcp</tt></td><td>Is TCP?</td></tr>
<tr><td><tt>udp</tt></td><td>Is UDP?</td></tr>
<tr><td><tt>ip.*</tt></td><td>IPv4 fields (see <tt>WINDIVERT_IPHDR</tt>)</td></tr>
<tr><td><tt>ipv6.*</tt></td><td>IPv6 fields (see <tt>WINDIVERT_IPV6HDR</tt>)</td></tr>
<tr><td><tt>icmp.*</tt></td><td>ICMP fields (see <tt>WINDIVERT_ICMPHDR</tt>)</td></tr>
<tr><td><tt>icmpv6.*</tt></td><td>ICMPV6 fields (see <tt>WINDIVERT_ICMPV6HDR</tt>)</td></tr>
<tr><td><tt>tcp.*</tt></td><td>TCP fields (see <tt>WINDIVERT_TCPHDR</tt>)</td></tr>
<tr><td><tt>tcp.PayloadLength</tt></td><td>The TCP payload length</td></tr>
<tr><td><tt>udp.*</tt></td><td>UDP fields (see <tt>WINDIVERT_UDPHDR</tt>)</td></tr>
<tr><td><tt>udp.PayloadLength</tt></td><td>The UDP payload length</td></tr>
</table>
</center>
</p><p>
A <i>test</i> also fails if the field is missing.
E.g. the test "<tt>tcp.DstPort == 80</tt>" will fail if the packet does not
contain a TCP header.
</p>

<a name="filter_examples"><h3>7.1 Filter Examples</h3></a>

<p>
<ol>
<li>
Divert all outbound web traffic:
<pre>
HANDLE handle = WinDivertOpen(
        "outbound and "
        "(tcp.DstPort == 80 or udp.DstPort == 53)",
        0, 0, 0
    );
</pre>
</li>
<li>
Divert all inbound TCP SYNs:
<pre>
HANDLE handle = WinDivertOpen(
        "inbound and "
        "tcp.Syn",
        0, 0, 0
    );
</pre>
</li>
<li>
Divert only (inbound) local traffic:
<pre>
HANDLE handle = WinDivertOpen(
        "inbound and ("
        "(ip.DstAddr &gt;= 127.0.0.1 and ip.DstAddr &lt;= 127.255.255.255) or"
        "ipv6.DstAddr == ::1)",
        0, 0, 0
    );
</pre>
</li>
<li>
Divert all traffic:
<pre>
HANDLE handle = WinDivertOpen("true", 0, 0, 0);
</pre>
</li>
<li>
Divert no traffic:
<pre>
HANDLE handle = WinDivertOpen("false", 0, 0, 0);
</pre>
This is useful for packet injection.
</ol>

<a name="filter_usage"><h3>7.2 Filter Usage</h3></a>

<p>
The purpose of the filter is to help applications select the subset of
all network traffic that the application is interested in.
Ideally the filter should be
<ol>
<li> As short as possible; and</li>
<li> As selective as possible.</li>
</ol>
For some applications these two objectives can conflict.
That is, a selective filter is not short, and a short filter is not selective.
For such applications the developer should experiment with different
filter configurations and carefully measure the performance impact to
find the optimal solution.
</p>

<hr>
<a name="samples"><h2>8. Samples</h2></a>

<p>
Some samples have been provided to demonstrate the WinDivert API.
The sample programs are:
<ul>
<li><tt>webfilter.exe</tt>: A simple URL blacklist filter.
    This program monitors outbound HTTP traffic.
    If it finds a URL request that matches the blacklist, it hijacks the
    TCP connection, reseting the connection at the server's end, and
    sending a simple block-page to the browser.
    The blacklist(s) are specified at the command-line.</li>
<li><tt>netdump.exe</tt>: A simple packet sniffer based on the
    WinDivert filter language.
    This program takes a filter specified at the command line, and prints
    information about any packet that matches the filter.
    This example uses WinDivert in "packet sniffing" mode, similar to
    <tt>winpcap</tt>.
    However, unlike <tt>winpcap</tt>, WinDivert can see local (loopback)
    packets.
<li><tt>netfilter.exe</tt>: A simple firewall based on the WinDivert
    filter language.
    This program takes a filter specified at the command line, and blocks
    any packet that matches the filter.
    It blocks TCP by sending a TCP reset, UDP by an ICMP message, and all
    other traffic it simply drops.
    This is similar to the Linux <tt>iptables</tt> command with the
    <tt>-j REJECT</tt> option.</li>
<li><tt>passthru.exe</tt>: A simple program that simply re-injects every
    packet it captures.
    This example is multi-threaded, where multiple threads are processing
    packets from a single handle.
    This example is useful for performance testing, and as a starting point
    for more interesting applications.</li>
</ul>
</p><p>
The samples are intended for educational purposes only, and are not
fully-featured applications.
</p><p>
The following basic template for a WinDivert application.
The basic idea is to open a WinDivert handle, then enter a
capture-modify-reinject loop:
<pre>
    HANDLE handle;          // WinDivert handle
    WINDIVERT_ADDRESS addr; // Packet address
    char packet[MAXBUF];    // Packet buffer
    UINT packetLen;

    handle = WinDivertOpen("...", 0, 0, 0);   // Open some filter
    if (handle == INVALID_HANDLE_VALUE)
    {
        // Handle error
        exit(1);
    }

    // Main capture-modify-inject loop:
    while (TRUE)
    {
        if (!WinDivertRecv(handle, packet, sizeof(packet), &amp;addr, &amp;packetLen))
        {
            // Handle recv error
            continue;
        }

        // Modify packet.

        if (!WinDivertSend(handle, packet, packetLen, &amp;addr, NULL))
        {
            // Handle send error
            continue;
        }
    }
</pre>
For applications that do not need to modify the packet, a better approach is
to open the WinDivert handle with the <tt>WINDIVERT_FLAG_SNIFF</tt> flag set,
and not re-inject the packet with
<a href="#divert_send"><tt>WinDivertSend()</tt></a>.
See the <tt>netdump.exe</tt> sample program for an example of this usage.
</p>

<hr>
<a name="known_issues"><h2>9. Known Issues</h2></a>

<p>
There are some limitations to the WinDivert package.
They are
<ul>
<li><i>Injecting inbound ICMP/ICMPv6 messages</i>:
    For some ICMP/ICMPv6 messages, inbound injection does not work.
    An error will be returned and the packet will be lost.
    The work-around is to inject inbound ICMP messages as <tt>outbound</tt>.
    </li>
<li><i>No IPv6 extension header support</i>:
    Currently there is no filter support for IPv6 packets with extension
    headers.
    The work around is to capture all IPv6 traffic.</li>
<li><i>The forward layer does not interact well with the Windows NAT</i>:
    It is not possible to block packets pre-NAT with WinDivert.
    As a general principle, you should not try and mix WinDivert at the
    forward layer with the Windows NAT implementation.
</ul>
</p>

<hr>
<a name="license"><h2>10. License</h2></a>
<p>
This package is distributed <i>strictly</i> under the
<a href="http://www.gnu.org/licenses/lgpl-3.0.txt">GNU Lesser General
Public License (GPL) Version 3</a>.
Please note the following:
<pre>
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License
along with this program.  If not, see &lt;http://www.gnu.org/licenses/&gt;.
</pre>
</p>
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